This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The design work on the high-power stage of a receiver protector switch for 35 GHz is completed. We are integrating the ferrite switch with a fast, low-power, pin-diode switch that we recently acquired. We successfully tested a ferrite-based latching circulator intended for 35 GHz work. It is controlled by a home-built, fast, pulsed current driver. The switching speed is better than 20 ns, which is adequate for its use in FT-ESR. The repetition rate for this switch is 5 kHz, but we have not attempted to increase it further due to lack of a spare switch. The remaining work also includes minimizing electromagnetic interference from intense current pulses in its normal mode of operation as a part of the 35 GHz spectrometer. We routinely use 200 W limiters to handle 1-1.5 kW pulses of nanosecond duration for 8-18 GHz frequency range. For operating frequencies up to 18 GHz at higher peak power we recently procured passive receiver protectors rated to 1 kW for 1 ?s pulses. This would be adequate for nanosecond pulses of higher intensity. Their recovery time of 50 ns is adequate for low-temperature DQC work, but may be problematic for 2D-ELDOR for which dead-time should be under 30 ns. This would require a further effort to procure 20 ns ferrite switches and combine them with low-power diode limiters in order to achieve dead-time less than 25 ns. Ferrite switches for X and Ku bands with switching time under 20 ns are not available as standard products. What can be found is specified for larger switching energy than the 35 GHz switch, therefore we plan to obtain samples and study their switching performance.